A related art of a clutch mechanism which connects/disconnects power is disclosed in Patent Literature 1. The clutch mechanism in Patent Literature 1 is a synchronous self-shifting clutch (SSS clutch), wherein a switch sleeve rotates at a rotation speed of a steam turbine until the switch sleeve reaches a synchronous rotation speed, and the switch sleeve is firmly held by a claw of a switch portion of a power generator shaft if the switch sleeve reaches the synchronous rotation speed. If the switch sleeve exceeds the synchronous rotation speed, the switch sleeve is moved in an axial direction in the direction of the steam turbine by a helical screw. After a short time, teeth of the switch sleeve and teeth of the power generator shaft engage with each other, and power is transmitted via the teeth. On the contrary, if the rotation speed of the steam turbine becomes lower than the synchronous rotation speed, the switch sleeve is moved in an axial direction movement in a direction opposite to the steam turbine by the helical screw, and the teeth of the switch sleeve and the teeth of the power generator shaft disengage from each other so that the transmission of the power is cut off.